Our long term goal is to understand the role of dysregulated Notch signaling in the pathogenesis of T cell leukemia and to develop new diagnostic, prognostic, and therapeutic strategies based on this information. Recently, Notch mutations have been associated with more than half of human T-ALL, suggesting that advances in understanding leukemogenic Notch signaling will have a significant impact on this disease. Over the last decade, we have developed mouse models, tissue culture assays, and biochemical methods to investigate the role of Notch signaling in T cell leukemia. Here, we will build on these findings to determine whether mutant forms of Notchl drive ectopic T cell development and induce leukemias in mice, and will correlate these outcomes with primary effects of various Notchl alleles on defined subsets of hematopoietic progenitors. Notchl appears to have a broad capacity to collaborate with a number of other transcription factors linked to human T-ALL development. We will determine the ability of these mutated Notch alleles to synergize with other genes commonly implicated in human T-ALL, such as Hox 11, Hox11L and Tall. Finally, Notchl's leukemogenic activity has been linked to up-regulation of unknown downstream target genes. Using a series of well-controlled expression profiling experiments and novel Notch-dependent T-ALL cell lines, we will identify downstream target genes that contribute to the sustained growth of T-ALL cells. Together, these studies will provide a deeper understanding of the pathogenesis of Notch-induced leukemia, and thereby move us closer to novel therapeutic interventions in this disease and other pathologic states characterized by aberrant Notch signaling. The specific aims of this proposal are: 1. To determine the transforming ability of the recently identified Notchl HD and/or PEST mutations 2. To determine how Notchl signals of varying strength interact with other transcription factors to induce TALL 3. To determine the mechanism(s) by which oncogenic Notchl promotes transformation